The present invention relates to a wide-angle arc segment magnet supported by a cylindrical yoke for constituting a magnetic circuit for motors, generators, etc., and more particularly to a wide-angle arc segment magnet whose magnetic flux density distribution has a smooth wave form substantially free from deformation in a circumferential direction approximately on an inner surface thereof, when a pair of them are assembled in the magnetic circuit, thereby generating extremely reduced magnetic noises. It further relates to a brush motor containing a pair of such wide-angle arc segment magnets.
Widely used in magnetic circuits of motors, generators, etc. are hard ferrite magnets, Alnico magnets, Mn-Al magnets, Fe-Cr-Co magnets, rare earth magnets (Sm-Co or Nd-B-Fe), etc. Such magnets are generally in the form of a cylinder or an arc obtained by dividing a cylinder, and magnetic anisotropy is given to such magnets in radial directions or in longitudinal directions.
As an arc segment magnet, for instance, Japanese Utility Model Laid-Open No. 59-78880 shows in FIG. 5 an anisotropic magnet having radial magnetic anisotropy, and Japanese Patent Laid-Open No. 2-119546 shows in FIG. 1 an anisotropic magnet having two magnetic poles opposing diametrically. These magnets are produced by different methods.
Magnets are generally produced by a dry molding method or a wet molding method. The dry molding method is considered to be more suitable for the production of magnets in that it takes shorter time than the wet molding method and that it can produce magnets having complicated shapes by utilizing a smaller apparatus. Accordingly, the arc segment magnets are conventionally produced by the dry molding method.
In motors, etc. using arc segment magnets, it is desired that as small magnetic noises as possible are generated in their operation. The magnetic noises are caused by the fact that the magnetic flux density distribution in a magnetic circuit of the motor has a deformed wave form. When there is a deformed wave form, torque generated by a magnetic attraction force between a stator and a rotor is periodically changed, causing a so-called cogging, which means uneven rotation. This cogging is a main cause for noises.
To avoid such magnetic noises, an eccentrically shaped arc segment magnet is used for motors. Further, Japanese Patent Laid-Open No. 63-228951 assigned to Hitachi Metals, Ltd. discloses an arc segment magnet having longitudinal ends each having a skewed shape, and an eccentrically shaped, inner cylindrical surface to stabilize the rotation of a rotor.
However, although the eccentrically shaped arc segment magnets generate reduced magnetic noises, their effective magnetic flux is also reduced since a space factor of the magnet is reduced due to eccentric disposition, meaning that a relative volume of the magnet is reduced. In addition, since sufficient orientation of magnetic anisotropy cannot be achieved in eccentrically shaped magnets, they do not show high magnetic properties even though they are shaped in the form of a ring.
Apart from the above, the dry molding method cannot easily produce an arc segment magnet having an arc angle .theta. close to 180.degree.. As shown in FIG. 10, an arc segment magnet is produced by using a die apparatus comprising an upper die 11 having a cylindrical recess, a lower die 12 having an aperture, a plunger 13 having a top surface provided with a cylindrical center projection and movable up and down in the aperture of the lower die 12. A cavity 16 in an arcuate shape is defined by the cylindrical recess of the upper die 11 and the top surface of the plunger 13. The magnet powder is charged into the cavity 16 and compressed by elevating the plunger 13. In this case, if an arc segment magnet to be produced has an arc angle close to 180.degree., it is difficult to remove the resulting green body from the die without destroying edge portions thereof. In addition, in a dry molding method in which a magnetic field is applied longitudinally, both circumferential ends of a wide-angle arc segment magnet cannot have a high density as compared to a center portion thereof. Accordingly, when a molded product (green body) is taken out of a die, it is likely to be cracked and broken at both circumferential ends. In a dry molding method in which a magnetic field is applied transversely (Japanese Patent Publication No. 56-51482), a wide-angle arc segment magnet cannot be easily removed from a die as an arc angle .theta. of the arc segment magnet increases. Accordingly, cracking is also likely to take place. For these reasons, arc segment magnets having arc angles .theta. of 150.degree.-165.degree. are generally produced.
On the other hand, in the case of a wet molding method using a magnet powder slurry, a large deformation is more likely to take place than in the case of the die molding method. Particularly, when a die apparatus having a similar structure to that shown in FIG. 10 is utilized, the density of the resulting green body is likely to be uneven. Specifically, a center portion of the resulting arc segment-shaped green body has a higher density, and side portions (circumferential end portions) have a lower density. Accordingly, the sintered arc segment magnet tends to have a large deformation.
Because of the above mentioned problems, a conventional arc segment magnet does not have an arc angle .theta. close to 180.degree.. Therefore, when assembled inside a cylindrical yoke of a motor, etc., the resulting arc segment magnet assembly has a large gap which leads to a slight deformation of a wave form of a magnetic flux density distribution. This causes vibration and noises.